1. Field of the Invention
The present invention relates to offshore modular and floatable gravity structures which are normally supported on the sea bed in shallow water and which, in deeper water include a steel gravity platform is supported by a concrete base resting on the ocean floor; with steel gravity structure being adapted to support an oil and/or gas exploration or production platform. More particularly, the invention is adapted for use in an arctic environment wherein the structural system is subjected to significant horizontal and tipping moments generated by impinging ice sheets, ice packs, and iceridges or floes.
2. Discussion of the Prior Art
Heretofore, a number of varied solutions to the problems encountered in protecting offshore oil and gas drilling structures from damage caused by ice sheets, ice packs, and iceridges or floes have been suggested in the prior art. This technology has developed as the offshore exploration and production of oil and gas has extended into arctic regions consisting of oceans, inlets and bays wherein the waters are frequently covered by vast sheets of ice during the winter months, and extremely large ice floes in the magnitudes ranging up to a mile across and even larger during and other seasons.
Pierce et al. U.S. Pat. No. 4,245,929 discloses an offshore structure which is able to withstand ice forces generated by impinging ice sheets, ice packs, or iceridges, and wherein the lower portion of the support structure of the offshore platform includes upper and lower differently sloped conical exterior wall portions to form an inclination relative to the horizontal. The inclined conical wall portions are designed to deflect ice masses coming into contact with the platform support structure. The particular structural selection of the conical wall structure is designed to cause the ice to tilt upwardly upon impinging against the support structure and fragment the ice by converting the horizontal load to vertical tensile stresses. In contrast therewith, the invention improves upon the structure disclosed in Pierce et al. in at least two major respects. Firstly, the inventive structure is modular and floatable to allow for repositioning of the structure when the system is used for exploratory oil and gas well drilling. Secondly, the structure is designed to generate extremely high gravitational shear forces which will withstand the horizontal and vertical forces normally generated by ice sheets and dense ice packs. Additionally, the gravity mass of the inventive structure is sufficient to withstand ocean waves of maximum amplitude for the depth of water in which the structure is intended to operate.
Howard U.S. Pat. No. 3,766,737 discloses an offshore platform which is encompassed, at a radial distance from the platform, by a circumferentially movable ice trenching machine. This machine circulates about the platform so as to fragment and remove ice in a circular path at a rate approximately equal to the rate of movement of the ice sheet.
Oshima et al. U.S. Pat. No. 4,230,423 discloses a rotary ice breaking member having spiral rotary blades attached to the main structure thereof for use in icy waters. The rotary blades raise the ice sheet or dense ice pack and cause it to shear or break in a flexural mode as the ice is raised.
Challine et al. U.S. Pat. No. 4,142,819 discloses an offshore platform in which the platform is of the gravity displacement type. This prior art structure includes a base member resting on the marine floor, and has an annular steel shell affording rigidity in the upwardly extending direction, and incorporates a circular wall and diaphragm extending about the base portion of the platform so as to constitute a reinforcement for the base structure. While Challine et al. disclose a portable drilling platform for use on the ocean floor, it is not particularly intended for use in the arctic environment, nor does it disclose any structure for protecting the device from the horizontal forces generated by sheet ice and dense ice packs.
Galloway U.S. Pat. No. 3,881,318 discloses a method and an apparatus for creating an artificial ice ridge to protect the work platforms from encroaching ice sheets, pressure ridges, ice floes and the like.